As is well known in the wood pulp bleaching art, the main objectives of wood pulp bleaching are to increase the brightness of the pulp and to make it suitable for the manufacture of printing and tissue grade papers by removal or modification of some of the constituents of the unbleached pulp, including the lignin and its degradation products, resins, metal ions, non-cellulosic carbohydrate components, and various types of flecks. The bleaching of chemical wood pulp is normally carried out in multiple processing stages utilizing elemental chlorine, caustic soda, hypochlorites, oxygen, hydrogen peroxide, and chlorine dioxide. The number of stages required in a particular bleaching process is dependant upon the nature of the unbleached pulp as well as the end use to which the pulp will be put. A sulfate or kraft pulp is today most typically bleached in a five stage sequence which is designated as (CD)(EO)DED. In the (CD)(EO)DED designation, D denotes chlorine dioxide, C denotes elemental chlorine, E denotes caustic extraction, and O denotes oxygen gas. The multi-stage process in essence comprises a chlorination step (CD), a first oxidative extraction stage (EO), a first bleaching stage (D.sub.1), a second caustic extraction stage (E.sub.2), and a second and final bleaching stage (D.sub.2).
In the conventional (CD)(EO)DED multi-stage bleaching process, each of the two chlorine dioxide bleaching stages is carried out in a one-step process at an end pH of about 3.8 for three hours at 70.degree. centigrade. It is commonly known that pH has an important bearing on brightness and strength properties as well as the chemical species present in the wood pulp mixture, and this particular pH has heretofore been considered optimal for each of the two chlorine dioxide bleaching stages in the (CD)(EO)DED sequence. It should also be appreciated that although the (CD)(EO)DED sequence has been specifically addressed, the one-step chlorine dioxide bleaching stage can be used in any D stage for most other three, four, five, or six-stage bleaching processes known to those familiar with the art of wood pulp bleaching.
A shortcoming of the one-step chlorine dioxide bleaching stage presently used in the pulp and paper industry is that approximately 30% of the chlorine dioxide is lost to the formation of the unreactive species chlorite and chlorate, and this is very undesirable in view of the relatively high cost of chlorine dioxide. The present invention solves this well-known deficiency in state of the art chlorine dioxide bleaching by significantly reducing the chlorine dioxide loss during the chlorine dioxide bleaching process. The advantages of the reduced loss of chlorine dioxide are a very significant reduction in the cost of the wood pulp bleaching process as well as the reduction of pollution levels.